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The Journal of Neuroscience, April 15, 2003, 23(8):3145
Circadian Phase-Dependent Modulation of cGMP-Gated
Channels of Cone Photoreceptors by Dopamine and D2 Agonist
Gladys Y.-P.
Ko,
Michael L.
Ko, and
Stuart E.
Dryer
Department of Biology and Biochemistry and Biological Clocks
Program, University of Houston, Houston, Texas 77204-5001
The affinity of cGMP-gated ion channels (CNGCs) for cGMP in chick
retinal cone photoreceptors is under circadian control. Here we report
that dopamine (DA) and D2 receptor agonists evoke phase-dependent
shifts in the affinity of CNGCs for activating ligand. Inside-out patch
recordings from cultured chick cones were performed at circadian time
(CT) 4-7 and CT 16-19 on the second day of constant darkness.
Exposing intact cells to DA or the D2 agonist quinpirole for 2 hr
before patch excision caused a significant increase in the
KD for cGMP during the night (CT 16-19) but
had no effect during the day (CT 4-7). DA or quinpirole treatment had
no effect on the Hill slope or the average number of channels per
patch. The effect of DA was blocked by the D2 antagonist eticlopride
and was not mimicked by D1 agonists or blocked by D1 antagonists. By
contrast, a brief (15 min) exposure to DA or quinpirole caused a
decrease in KD during the subjective day and
had no effect during the subjective night. Thus, the effect of D2
agonists depends on both the duration of agonist exposure and the time
of day. Application of DA or quinpirole evoked a transient activation
of the MAP kinase Erk (extracellular signal-related kinase)
during the day but caused a sustained inhibition during the night.
Conversely, D2 agonists caused activation of
Ca2+/calmodulin-dependent protein kinase II during
the night and inhibited this enzyme during the day. A circadian
oscillator in cones appears to regulate the nature of the transduction
cascade used by D2 receptors.
Key words:
CNG channels; photoreceptor; cone; dopamine; circadian rhythm; D2 receptor
Copyright © 2003 Society for Neuroscience 0270-6474/03/2383145-09$05.00/0
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